Conventionally the No. 4 receiver has a metal front and rear cover. Inside the front cover is a membrane that excludes moisture, a damping disc and a frustoconical diagraphm. A connecting rod fixed to the diaphragm is connected at its other end to the rocking armature of an electromagnetic driving system that includes a pair of series-wound coils between which is located a permanent magnet and a yoke of soft magnetic material that passes into the coils and also provides a fixing for the driving system. The driving system and electrical connections for the coils are secured to an annular metal frame that fits into the front cover behind the diaphragm and which provides a seating for the rear cover which is crimped or spun over the front cover. The construction of the No. 3 receiver is similar but includes no membrane or separate damping disc. The frame that carries the driving system accounts for a considerable proportion of the cost of the transducer.